1. Field of the Invention
The present invention relates to a color cathode ray tube in which three electron beams of an in-line arrangement passing on the same plane are emitted and, more particularly to a color cathode ray tube in which the three electron beams of an in-line arrangement are preferably focused and converged on a phosphor screen.
2. Description of the Related Art
In general, a color cathode ray tube has an envelope constituted by a panel and a funnel integrally connected to the panel. A phosphor screen constituted by three phosphor layers for emitting blue, green, and red light rays is formed on the inner surface of the panel, and a shadow mask having a plurality of apertures is formed inside the phosphor screen to be opposite to the phosphor screen. An electron gun assembly for emitting three electron beams is sealed in a neck of the funnel. The three electron beams are deflected by horizontal and vertical deflection magnetic fields generated by a deflection apparatus arranged outside the funnel, and the three electron beams are incident on the phosphor screen through the shadow mask, thereby displaying a color image.
An example of an electron gun assembly for the in-line color cathode ray tube is as follows. Three electron beam through-holes of an electrode arranged on the phosphor screen side form a main electron lens portion. Electron beam through-holes, through which a pair of side beams pass, are offset outside the arrangement direction of the three electron beams to a greater extent than those of an electrode arranged on the cathode side, opposite to the electrode on the screen side. Thus, the three electron beams converge. As an electron gun assembly capable of solving the problems of the above electron gun assembly, an electron gun assembly having the following arrangement is used. That is, the three electron beam through holes are coaxially formed, each of a plurality of electrodes arranged on cathodes. An annular permanent magnet for generating a quadrupole component magnetic field having both a focusing function and a diverging function is arranged on the final electrode, so that the three parallel electron beams emitted from the electron gun assembly are converged on a phosphor screen by a magnetic quadruple lens formed by the magnet in the arrangement direction of three electron beams, the diverging function is in the direction perpendicular to the arrangement direction.
According to this electron gun assembly, since the three electron beam through-holes of each of the plurality of electrodes arranged on the cathodes can be coaxially formed, the assembling accuracy of the electron gun assembly can be improved. The assembling accuracy is one of the problems of the electron gun assembly in which the electron beam through holes, through which the pair of side beams pass, are offset outside the three electron beams in the arrangement direction of the three electron beams more largely than those of an electrode opposite to the electrode arranged on the screen side. However, a high-power magnetic field is required to converge the three parallel electron beams on the phosphor screen by the quadrupole component lens formed by the annular permanent magnet. Due to the high-power magnetic field, the shapes of the beam spots are considerably distorted, thereby degrading a resultant image.